But a continuing high-altitude freeze over the Arctic may have already reduced ozone to half its normal concentrations—and "an end is not in sight," said research leader Markus Rex, a physicist for the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany.

Preliminary data from 30 ozone-monitoring stations throughout the Arctic show the degree of ozone loss was larger this winter than ever before, Rex said.

Before spring is out, "we may even get the first Arctic ozone hole ... which would be a dramatic development—one which would make it into coming history books," he said.

"It's too early to call, but stay tuned."

Atmospheric chemist Simone Tilmes, who wasn't part of the study, agreed.

"We do not know at the moment how large the ozone hole in the Arctic will grow, because the thinning of the ozone layer is happening right now," said Tilmes, of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.

Full confirmation may require computer simulations and satellite measurements, which study leader Rex said would "be very useful to provide an independent view of the ozone loss this year."

An ozone hole is an area of the ozone layer that is seasonally depleted of the protective gas—such as the well-known hole over Antarctica.

In the 1980s scientists realized chlorofluorocarbons (CFCs) and other ozone-depleting chemicals—then widely used in aerosol hairsprays and refrigerants, for example—were degrading the ozone layer.

The 1987 Montreal Protocol initiated a global phase-out of CFCs, replacing them with alternatives that don't destroy ozone. However, CFCs can persist for decades in the stratosphere—the Antarctic ozone hole is still there, though it's expected to grow smaller in coming decades.

Once in the upper atmosphere, CFCs break down into chlorine atoms, which, when activated by sunlight, destroy ozone molecules.

The clouds provide "reservoirs" for inactivated byproducts of chlorine. On the surface of the cloud, these byproducts react with each other and release "aggressive" chlorine atoms that attack ozone molecules.

The whole process stops as soon as it gets warmer and the so-called Arctic polar vortex breaks up, Tilmes said.

At about 6 million square miles (15 million square kilometers), or 40 times the size of Germany, the Arctic polar vortex is a frigid air mass that circles the North Pole in winter.

Warming Link to High-Altitude Cold Snap?

The cold snap is no coincidence, research leader Rex added.

"This is the continuation of a long-term tendency that the cold Arctic winters have become colder," Rex said.

And global warming may drive this trend, he added. As greenhouse gases trap heat in the lower levels of the atmosphere, the higher levels tend to cool, he said.

Of course, the "process is more complicated than this simple explanation"—there may be many ways in which greenhouse gases influence high-altitude temperatures, he added.

Low-Ozone Air to Fly South for Spring?

Any spike in UV radiation can impact both the Arctic ecosystem and human health, research leader Rex noted. For instance, more sunlight can slow the growth of certain species of ocean algae that provide food for larger organisms—and whose absence can have reverberations up the food chain.